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.\" ========================================================================
.\"
.IX Title "SSL_CONNECT 3ossl"
.TH SSL_CONNECT 3ossl "2026-01-16" "3.5.1" "OpenSSL"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH "NAME"
SSL_connect \- initiate the TLS/SSL handshake with an TLS/SSL server
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& #include <openssl/ssl.h>
\&
\& int SSL_connect(SSL *ssl);
.Ve
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\fBSSL_connect()\fR initiates the \s-1TLS/SSL\s0 handshake with a server. The communication
channel must already have been set and assigned to the \fBssl\fR by setting an
underlying \fB\s-1BIO\s0\fR. \fBssl\fR \fB\s-1MUST NOT\s0\fR be \s-1NULL.\s0
.SH "NOTES"
.IX Header "NOTES"
The behaviour of \fBSSL_connect()\fR depends on the underlying \s-1BIO.\s0
.PP
If the underlying \s-1BIO\s0 is \fBblocking\fR, \fBSSL_connect()\fR will only return once the
handshake has been finished or an error occurred.
.PP
If the underlying \s-1BIO\s0 is \fBnonblocking\fR, \fBSSL_connect()\fR will also return
when the underlying \s-1BIO\s0 could not satisfy the needs of \fBSSL_connect()\fR
to continue the handshake, indicating the problem by the return value \-1.
In this case a call to \fBSSL_get_error()\fR with the
return value of \fBSSL_connect()\fR will yield \fB\s-1SSL_ERROR_WANT_READ\s0\fR or
\&\fB\s-1SSL_ERROR_WANT_WRITE\s0\fR. The calling process then must repeat the call after
taking appropriate action to satisfy the needs of \fBSSL_connect()\fR.
The action depends on the underlying \s-1BIO.\s0 When using a nonblocking socket,
nothing is to be done, but \fBselect()\fR can be used to check for the required
condition. When using a buffering \s-1BIO,\s0 like a \s-1BIO\s0 pair, data must be written
into or retrieved out of the \s-1BIO\s0 before being able to continue.
.PP
Many systems implement Nagle's algorithm by default which means that it will
buffer outgoing \s-1TCP\s0 data if a \s-1TCP\s0 packet has already been sent for which no
corresponding \s-1ACK\s0 has been received yet from the peer. This can have performance
impacts after a successful TLSv1.3 handshake or a successful TLSv1.2 (or below)
resumption handshake, because the last peer to communicate in the handshake is
the client. If the client is also the first to send application data (as is
typical for many protocols) then this data could be buffered until an \s-1ACK\s0 has
been received for the final handshake message.
.PP
The \fB\s-1TCP_NODELAY\s0\fR socket option is often available to disable Nagle's
algorithm. If an application opts to disable Nagle's algorithm consideration
should be given to turning it back on again later if appropriate. The helper
function \fBBIO_set_tcp_ndelay()\fR can be used to turn on or off the \fB\s-1TCP_NODELAY\s0\fR
option.
.SH "RETURN VALUES"
.IX Header "RETURN VALUES"
The following return values can occur:
.IP "0" 4
The \s-1TLS/SSL\s0 handshake was not successful but was shut down controlled and
by the specifications of the \s-1TLS/SSL\s0 protocol. Call \fBSSL_get_error()\fR with the
return value \fBret\fR to find out the reason.
.IP "1" 4
.IX Item "1"
The \s-1TLS/SSL\s0 handshake was successfully completed, a \s-1TLS/SSL\s0 connection has been
established.
.IP "<0" 4
.IX Item "<0"
The \s-1TLS/SSL\s0 handshake was not successful, because a fatal error occurred either
at the protocol level or a connection failure occurred. The shutdown was
not clean. It can also occur if action is needed to continue the operation
for nonblocking BIOs. Call \fBSSL_get_error()\fR with the return value \fBret\fR
to find out the reason.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBSSL_get_error\fR\|(3), \fBSSL_accept\fR\|(3),
\&\fBSSL_shutdown\fR\|(3), \fBssl\fR\|(7), \fBbio\fR\|(7),
\&\fBSSL_set_connect_state\fR\|(3),
\&\fBSSL_do_handshake\fR\|(3),
\&\fBSSL_CTX_new\fR\|(3)
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright 2000\-2020 The OpenSSL Project Authors. All Rights Reserved.
.PP
Licensed under the Apache License 2.0 (the \*(L"License\*(R").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file \s-1LICENSE\s0 in the source distribution or at
<https://www.openssl.org/source/license.html>.
